1. Field of the Invention
The present invention relates to a laser welding method by which two members are stacked together (abutted against each other) and an edge of the abutting surfaces of the two members is irradiated with a laser beam, which is directed toward a deeper location between the abutting surfaces, so that the abutting surfaces of the two members are laser welded together at the desired penetration depth (depth of penetration). The present invention also relates to a method for manufacturing a welded body.
2. Description of the Related Art
Two members may be laser-welded together (hereinafter also referred to simply as welded together) by, for example, arranging one principal surface (flat surface) of one member (plate member) and one flat surface of the other member, which may be a rectangular parallelepiped, so that the surfaces abut against each other and welding the abutting surfaces (opposing surface) together. In this case, one of the sides among the edges (outer edges) of the abutting surfaces (hereinafter referred to simply as one side) is irradiated with a laser beam directed toward a deeper location between the abutting surfaces. The one side, which serves as a welding line, is scanned with the laser beam at a constant velocity, so that the abutting surfaces of the two members (base materials) are joined together. This process is carried out to manufacture, for example, a ground electrode included in a spark plug of an automobile engine (see Patent Document 1). To increase the welding strength (joining strength) while ensuring sufficient precision in the welding process, the abutting surfaces need to be melted and solidified over as large an area as possible within a range in which excessive melting does not occur. For this purpose, parameters such as the output and scanning velocity (irradiation time) of the laser beam need to be set so that the metals are melted to a necessary and sufficient depth of penetration (melting depth in the irradiation direction) along the abutting surfaces from the one side among the edges of the abutting surfaces, the one side being a light-receiving portion irradiated with the laser beam (incident side), toward the opposite side at a deeper location in the irradiation direction (side toward which the laser beam is directed).
FIGS. 6A to 6C are enlarged views illustrating an example of such a welding process. More specifically, FIGS. 6A to 6C are enlarged schematic views illustrating the process of welding a noble metal tip 20 onto a front portion of a ground electrode body 10 (only a portion thereof is illustrated) in the process of manufacturing a ground electrode (component) 31 included in a spark plug 100 illustrated in FIG. 8. FIG. 8 is a longitudinal half-sectional view of the spark plug 100 according to the related art. The spark plug 100 includes a metal shell 40 having a tubular shape including portions of different diameters; an insulator 50 having a hollow cylindrical shape that extends through the metal shell 40; a center electrode 60 that is disposed in the central axial hole in the insulator 50 and that has a tip exposed at a front end (top end in FIG. 8) 53 of the insulator 50; and a ground electrode 31 that is securely welded to a front end surface 43 of the metal shell 40 and includes a ground electrode body 10 that is bent. The noble metal tip 20, which is securely welded to a front end of the ground electrode body 10, and the tip of the center electrode 60 define a spark discharge gap. The noble metal tip 20 is provided to improve the discharge ignitability between the ground electrode 31 and the tip end of the center electrode 60, which is a counter electrode in the spark plug 100, and to increase durability. In this specification, the ground electrode and the center electrode may also be referred to simply as electrodes, and the ground electrode body and the center electrode body may also be referred to simply as electrode bodies.
Referring to FIGS. 6A to 6C, the noble metal tip 20 is welded onto the front portion of the ground electrode body 10 by the following process. First, the noble metal tip 20 (a small rectangular parallelepiped made of a noble metal, such as platinum or iridium, or an alloy having a noble metal as a main component) is placed on a plate surface (flat surface) 13 of the ground electrode body 10 (for example, a strip-shaped rectangular bar member made of a nickel alloy) in a region near a front end surface 11 of the ground electrode body 10. As illustrated in FIG. 6A (front view) and FIG. 6B (plan view), the noble metal tip 20 is positioned so as to be aligned with (or be in the vicinity of) the front end surface 11 of the electrode body 10. Then, among the edges of abutting surfaces 10a and 20a of the two members 10 and 20 (members to be welded together), a right edge 20e (edge of the noble metal tip) in FIG. 6B (plan view), that is, a side (welding line WL) at the front end surface 11 of the ground electrode body 10, for example, is irradiated with a laser beam La and scanned from one end S1 toward the other end S2 at a constant velocity. Thus, the abutting surfaces 10a and 20a of the two members are welded together. The ground electrode body 10 is a small rectangular bar member (thin strip-shaped member) having a width W1 of about 2 to 3 mm and a thickness H1 of about 1 to 1.5 mm in cross section. The noble metal tip (hereinafter also referred to as a tip) 20 welded to the ground electrode body 10 is a small rectangular parallelepiped having a thickness H2 of about 0.4 to 1 mm and a flat surface (see FIG. 6B) with a length and a width of about 1 to 1.5 mm. Therefore, the welding line WL is as short as about 1 to 1.5 mm. In the welding process, as described above, the welding line WL is scanned with the laser beam La once at a constant velocity from one end (welding start position) S1 to the other end (welding end position) S2, that is, for example, from the bottom end to the top end in FIG. 6B.
To achieve the desired high welding strength without reducing the precision in the above-described welding process, as described above, the penetration depth needs to be sufficient relative to the side length Lh of the noble metal tip 20 in the irradiation direction of the laser beam (direction from right to left in FIGS. 6A and 6B) without causing excessive melting of the metals along the abutting surfaces 10a and 20a, that is, in the area of the flat surface of the noble metal tip 20 in the plan view of FIG. 6B. The penetration depth also needs to be prevented from greatly varying along the side (welding line WL) from the start position at one end S1 of irradiation (hereinafter referred to also as a start position) to the end position at the other end S2 in the scanning direction of the laser beam La. Thus, the penetration depth is required to be within a penetration depth range (side length Lh±α) around the side length Lh with a small error α so that the penetration depth is sufficient relative to the side length Lh of the noble metal tip 20 and does not greatly vary along the welding line WL. This is because the ground electrode of the spark plug is subjected to severe conditions and required to have high ignitability and durability, and the abutting surfaces 10a and 20a thereof are required to be reliably welded over the entireties (entire areas) thereof without reducing the precision. Accordingly, the output, scanning velocity, etc., of the laser beam with which a necessary and sufficient penetration depth can be obtained have conventionally been determined based on test welding, and laser-beam scanning has been performed at a constant output and a constant velocity in the welding process. The laser-beam scanning can be performed not only by moving the laser beam but also by moving the members to be welded together (base materials) or by moving both the laser beam and the base materials relative to each other. These cases are also included in the laser-beam scanning described in this specification.
In the laser welding process, the irradiation laser beam has conventionally been set to a high constant output. The reason for this is as follows. In the state after the irradiation with the laser beam and melting of the base materials to be welded together (the two members, which are the ground electrode body and the noble metal tip in FIGS. 6A to 6C) have been started, the base materials have already been heated in a region around the scanned location. However, when the irradiation with the laser beam is started (when welding is started) at the start position S1, the base materials that are cold and in a solid state are irradiated with the laser beam and melted. Therefore, the thermal energy required to melt the base materials in this state is greater than that required to melt the base materials in a region around the melted portion in the state after the irradiation and scanning with the laser beam and melting of the base materials have been started (workpieces after the start of irradiation). Thus, in the region around the melted portion of the base materials in the state after the scanning and melting have been started, the base materials have already been heated and therefore the desired penetration depth can be obtained by a laser beam set to a relatively low output. In contrast, at the start position S1 where the base materials have not been heated by irradiation and melted, the desired penetration depth cannot be obtained unless the output of the laser beam is increased because the base materials have not yet been heated and melted. This is the reason why the laser beam has been set to a high output in the welding process according to the related art in which the base materials are irradiated and scanned with the laser beam set to a constant output.